British engineer Stephen Wallis has just shattered a speed barrier with his radio-controlled car, called Mach Reaper. His RC car passed 250 mph not once but twice, breaking his own record within a two-week span and becoming the fastest on Earth.
Wallis completed Project 250 – his eight-month engineering effort aimed at breaking the 250 mph barrier – with an officially timed run of 250.67 mph (403.4 km/h) and a GPS-recorded peak of 251.78 mph (405.1 km/h) at a ROSSA Radio Operated Scale Speed Association) event at Llanbedr Airfield in North Wales. Two weeks later, at the Yes Mate Speed Event on the same runway, he pushed Mach Reaper to a recorded GPS speed of 256.47 mph (412.7 km/h).
I Thought 250mph Was The Limit…
That’s not a toy hobby project anymore but a miniature exercise in aerospace-grade electrical engineering. Every part of Mach Reaper was designed from scratch, using computer-aided design (CAD) and computational fluid dynamics (CFD), the same simulation tools engineers use to model how air flows around full-size vehicles. The result is a four-motor electric drivetrain built entirely from custom-machined parts and bespoke electronics.
This isn’t Wallis’s first rodeo. He previously built The Beast, which set the Guinness World Record at 234.7 mph (377.7 km/h). Where most RC speed builders rely on complex transmissions and steering systems, Wallis’s original idea was to take a quadcopter drone (a small aircraft that flies using four spinning propellers) and replace its vertical propellers with horizontal wheels, turning a flying machine into a ground racer. That same obsession with stripping out complexity carries through Mach Reaper, though the new car is less a copy of the drone trick and more its evolution. Less clutter, sharper engineering, built to go even faster.
Stephen Wallis
Ahead of the new record-breaking run, Wallis had just five weeks to rework three systems: batteries, motors, and wheels. He wanted to switch to a higher-voltage battery configuration (referred to as “18s,” meaning 18 battery cells wired in series), simplify the wiring, and prepare the car for more power. More voltage also means less clutter and less dead weight, which translates to more speed.
One of the more effective changes involved swapping heavy heat sinks for custom motor controller plates, saving roughly 600 grams (1.3 lb) and freeing up space in the car’s center.
He also addressed a weak link in the drivetrain. One of his four motors had been rewound and ran slightly weaker than the rest. Replacing it with a matched motor eliminated a small but critical performance gap. At these speeds, even minor mismatches between components can be the difference between a record and a failed run.
Stephen Wallis
But going that fast brings its own set of problems to the car. Heat buildup increases with the square of electrical current, so Wallis ramped up power more gradually to avoid overheating rather than pushing full throttle immediately. He also tested active cooling with a fan positioned between the motor and wheel but ditched it after finding it didn’t help enough to justify the added weight and complexity. Instead, he refined how power was delivered, spending less time at current spikes that generate the most heat.
Tires posed a separate challenge. Standard tires weren’t built for a wheel this large, so Wallis found a way to shave them down more efficiently, reducing stress on the foam inside while shrinking the wheel diameter from 99 mm (3.9 in) to 94 mm (3.7 in). That smaller diameter improved traction, which mattered as much as raw power once Mach Reaper crossed the 240 mph (386 km/h) mark.
Stephen Wallis
The record runs themselves weren’t staged in ideal conditions. Wallis described tailwinds and even rain reducing visibility during the attempt, yet the car still delivered a clean pass.
Having already smashed his 250 mph target, Wallis isn’t done. He plans to keep pushing the record for the rest of the season. Mach Reaper has proven the ceiling was higher than expected, the real question now is how much higher it can go before physics finally says nay.
Source: Stephen Wallis
